Ultrasound machines are often used for observing organs in the human body. Typically, these machines contain transducer arrays for converting electrical signals into pressure waves or vice versa. Generally, the transducer array is in the form of a hand-held probe which may be adjusted in position to direct the ultrasound beam to the region of interest. As seen in FIGS. 1, 2 and 4, a transducer array 10 may have, for example, 128 transducer elements 12 in the azimuthal direction for generating an ultrasound beam. Adapted from radar terminology, the x, y, and z directions are referred to as the azimuthal, elevation, and range directions, respectively.
The transducer element 12, typically rectangular in cross-section, may comprise a first electrode 14, a second electrode 16, a piezoelectric layer 18, and one or more acoustic matching layers 20, 22. The transducer elements 12 are disposed on a backing block 24. In addition, a mechanical lens 26 may be placed on the matching layers to help confine the generated beam in the y-z plane. Examples of prior art transducer structures are shown in Charles S. DeSilets, Transducer Arrays Suitable for Acoustic Imaging, Ph.D. Thesis, Stanford University (1978) and Alan R. Selfridge, Design and Fabrication of Ultrasonic Transducers and Transducer Arrays, Ph.D. Thesis, Stanford University (1982).
Individual elements 12 can be electrically excited by electrodes 14 and 16, with different amplitudes and phases to steer and focus the ultrasound beam in the x-z plane. Terminals 28 and 30 may be connected to each of the electrodes 14 and 16 for providing the electrical excitation of the element 12. Terminal 28 may provide the hot wire or excitation signal, and terminal 30 may provide the ground. As a result, a primary wave 31 is provided in the z-direction.
The force distribution of the face 32 of the transducer element 12, and the acoustic and geometrical parameters of the mechanical lens 26 describe the radiation pattern in the elevation direction, as a function of an angle in the y-z plane. The finite width of the transducer element 12 in the y-direction causes the sides 36 and 38 of the transducer element 12 to move freely. This motion, in turn, creates lateral waves 40, propagating along the y-direction. These lateral waves 40, propagating through the composite structure of piezoelectric layer 18 and matching layers 20 and 22, may have a phase velocity greater than that of the external medium (e.g., the patient being examined) and may excite an undesirable secondary propagating wave and "leak" into the external medium.
The direction of the secondary propagating wave in the external medium is given by the expression .THETA.=arcsin(vo/vl), where .THETA. is measured with respect to the normal of the transducer face 32 in the y-z plane, vo is the velocity of the wave in the acoustic medium, and vl is the velocity of the lateral wave. This "leaky" wave will increase the sidelobe levels around the angle .THETA.. As an example, for the piezoelectric material PZT-5H, the phase velocity of the lateral wave is approximately 3000 meters per second. This is approximately twice the phase velocity in the human body of 1500 meters per second. Consequently, a secondary wave 42 caused by lateral wave 40 propagates at an angle .THETA. of 30 degrees.
The sidelobe levels of individual elements of an ultrasound transducer are of particular concern in applications where a strong reflector in the object of interest, e.g., cartilage, may be located outside the main acoustic beam. In such a case, the reflections from the object of interest, e.g., soft tissue, may be comparable to signals coming from a strong reflector, such as the cartilage, outside the region of interest. As a result, the generated image is less accurate and may contain artifact.
Referring also to FIG. 3, the main lobe of a typical ultrasonic transducer radiation pattern 44 is shown. Due to the contribution of lateral waves, the radiation pattern outlined by region 46 results. In the absence of the lateral wave, the radiation pattern would have followed curve 48. The radiation pattern 44 of a transducer is primarily related to the field distribution across its aperture. For continuous wave or very narrow band excitations, the radiation pattern is related to the aperture function by Fourier transform relationships. For wide band excitation, one may use, for example, superposition to integrate the field distributions at each frequency.
A fixed-focus lens may scale the radiation pattern by modifying the phase of the aperture distribution, but the general sidelobe characteristics are governed by the amplitude distribution of the aperture. In addition, apodization may be used to improve the radiation pattern by shaping the aperture distribution. Apodization results in varying the electric field between electrodes 14 and 16 along the elevation direction. However, these prior art techniques fall short because lateral waves still may be generated and contribute to undesirable sidelobe levels and may result in a less accurate image.